1. Field of the Invention
The present invention relates to a hydraulic automatic tensioner to be applied to a power transmitting mechanism using a transmission member, such as an endless belt or chain, for driving engine accessories of an internal combustion engine, and a power transmitting mechanism.
2. Description of the Related Art
A prior art hydraulic automatic tensioner applied to a power transmitting mechanism including a transmission member is disclosed in, for example, JP2000-220708A. FIGS. 18 and 19 shows the basic construction of this prior art hydraulic automatic tensioner in schematic views.
This hydraulic automatic tensioner includes a first member 01 having the shape of a bottomed cylinder and provided with a connecting lug 01a, and a second member 02 having the shape of a bottomed cylinder and provided with a connecting lug 02a. The first member 01 and the second member 02 are combined so as to be slidable relative to each other and to define a fluid-reservoir chamber 03. A spring 05 is extended between the first member 01 and the second member 02 so as to push the members 01 and 02 away from each other. A piston 06 formed integrally with the first member 01 is fitted in a cylinder 07 formed integrally with the second member 02 to define a pressure chamber 04.
The pressure chamber 04 communicates with the fluid-reservoir chamber 03 by means of a connecting passage 08 extending along the bottom surface of the second member 02. The connecting passage 08 is provided with a check valve 09 that permits the flow of the hydraulic fluid from the fluid-reservoir chamber 03 into the pressure chamber 04 and prevents the return flow of the hydraulic fluid. A leak passage 010 is formed between the pressure chamber 04 and the fluid-reservoir chamber 03 along the bottom surface of the second member 02, and an orifice is formed in the leak passage 010.
The connecting lug 01a (02a) is attached to a fixed member, and the connecting lug 02a (01a) is used for applying pressure to an endless belt to tighten the endless belt properly. When the first ember 01 and the second member 02 are moved toward each other, i.e., when the automatic tensioner is contracted, as shown in FIG. 18 to reduce the tension of the endless belt, the check valve 09 is closed by the pressure in the pressure chamber 04, and the hydraulic fluid leaks gradually from the pressure chamber 04 through the leak passage 010 provided with the orifice into the fluid-reservoir chamber 03. Thus, the first member 01 and the second member 02 move toward each other, i.e., the automatic tensioner contracts in order that the endless belt may not be excessively tensioned.
When the first member 01 and the second member 02 are moved away from each other, i.e., when the automatic tensioner is extended, by the resilience of the spring 05 to tension the endless belt, the pressure chamber 04 expands and the check valve 09 opens the connecting passage 08. Consequently, the hydraulic fluid flows rapidly from the fluid-reservoir chamber 03 into the pressure chamber 04 because the connecting passage 08 exerts low resistance against the flow of the hydraulic fluid. Thus, the first member 01 and the second member 02 are moved rapidly away from each other to tension the endless belt properly so that power can be surely transmitted.
When this prior art hydraulic automatic tensioner is applied to a power transmitting mechanism using a belt for driving engine accessories of an internal combustion engine, it occurs sometimes that a high tension is exerted first on the belt, and then the tension decreases suddenly.
In such a case, the first member 01 and the second member 02 of the prior art hydraulic automatic tensioner are moved toward each other so that the automatic tensioner contracts when a high tension is applied to the belt in an initial stage. Consequently, the belt cannot be properly tensioned and the belt is unable to transmit power securely. Furthermore, the first member 01 and the second member 02 are unable to move away from each other, i.e., the automatic tensioner is unable to extend, according to the subsequent sudden decrease of the tension, the belt slackens, the belt slips relative to the pulleys and, consequently, the belt is unable to transmit power properly.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a hydraulic automatic tensioner capable of properly coping with the sudden change of the tension of a transmission member and a special operating condition, and of always ensuring reliable power transmission.